We can’t say no, but we don’t understand sentience enough to say yes for certain. But, we can operate on the assumption that sentience and consciousness exists in humans and maybe in some other animals, while also knowing for certain that the word-generating program is only mimicking consciousness.

Can you though? You definitely can’t know that living things with brains aren’t to some significant degree responding to stimulus in a probabilistic way, and in the case of humans at least turning that response back on itself - essentially responding to itself. We do know for certain that the brain does a whole lot of that in variety of situations - it’s why optical illusions work, for example. It’s one of the brain’s favorite magic tricks - generating answers in a probabilistic way if there’s insufficient information, or sometimes if the information is inconvenient.